For every subscriber of a customary mobile communication system, a data record is stored in a data base denoted as home location register (HLR). Data records of different subscribers can be located in different home location registers. The data records comprise parameters which are used by control nodes in the communication system for the establishment of connections with user equipment of the subscriber. Connections can be established from the user equipment (user originating connection) or to the user equipment (user terminating connection). A connection can be for example a data connection, a speech call, an SMS (Short Message Service) message or the activation, deactivation or interrogation of a supplementary service like activating a call forwarding to a different subscriber number.
To reduce the signaling load during call set up and avoid high local computational loads, many communication systems comprise also visitor location registers (VLR). A VLR also contains sets with subscription related parameters for processing connections of the users served, generally those users located in a limited part of the area covered by the communication system. In most cases, the VLR is collocated with or part of the control node processing connections for the users served by the VLR, e.g. a mobile switching center (MSC). The VLR serves users by providing parameters for connections so that the connections can be processed by the control node serving the user without the necessity to contact the HLR.
Subscription related parameters which are used to process connections can be transferred by messages from the subscriber record in the HLR to the serving VLR. It is possible to copy all or a selection of the parameters since the parameters stored in the VLR can vary according to the capabilities of the VLR and the control node serving the corresponding area. The parameters specify, for example, either services that are provided to the user equipment or parameters which are used in connections. According to G-SM (Global System for Mobile Communications) specification 9.02 and UMTS (Universal Mobile Telecommunication System) 3G Technical Specification 29.002, V3.2-0 of the 3rd Generation Partnership Project, subscriber data is sent from the HLR to the VLR with the MAP (Mobil Application Part) message “Insert Subscriber Data” in response to the message “Update Location” from the VLR. The VLR provides the HLR with the present location of the user equipment. The “Update Location” message is generated by the VLR when the user equipment enters the region served. Especially in the event of a failure in a location register, subscriber data can also be requested with the message “Send Parameters” to update damaged, incomplete or missing subscriber records.
For locating user equipment within the communication system and routing connections to its present location, an identification of the visitor location register serving the user equipment is stored in the home location register. Users can enter the service area of a VLR by moving connected or idle equipment into the area from an area served by another VLR or by switching on the user equipment in the area served. If the presence of a new user equipment is detected in the service area of a VLR, a location update is performed for the subscriber. In a location update, the HLR is notified of the serving VLR which receives in return subscription related parameters from the subscriber record. For setting up a user terminating connection from a first control node, the node identifies the corresponding home location register from the subscriber number. Preferably, the HLR requests a temporary mobile subscriber roaming number from the control node presently serving the user equipment and forwards it to the first control node for setting up the connection. Alternatively, it is conceivable that the home location register provides the identity of the control node or VLR presently serving the subscriber to the first control node.
A location update causes processor load both in the new VLR and in the HLR to which the user equipment is assigned and generally also in a former VLR if a location cancellation is performed there. If the subscriber does not set up or receive a connection during his presence in the respective area, the location-update does not provide any benefit for the operator of the communication system. In many cases, location-updates without any subsequent connection by the user equipment in the respective area, represent a considerable percentage of all location-updates performed. An example for this situation is a densely populated area served by two or more VLRs where users frequently commute between the service areas. Another example are operators with a high percentage of subscribers who set up few connections which is a common behavior for subscribers with prepaid subscriptions.
If a restart of a VLR occurs, e.g. after a failure, stored data of some or all served users can be totally or partially lost or corrupted. In this case, the operation of the VLR is blocked for a considerable time by updating the parameter sets for the respective users in the VLR. During this considerable interval of time, the communication system cannot be accessed by users in the area served by the VLR.